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http://peeterjoot.com/2018/09/25/phy2403h-quantum-field-theory-lecture-5-klein-gordon-equation-hamiltons-equations-shos-momentum-space-representation-raising-and-lowering-operators-taught-by-prof-erich-poppitz/
PHY2403H Quantum Field Theory. Lecture 5: Klein-Gordon equation, Hamilton's equations, SHOs, momentum space representation, raising and lowering operators. Taught by Prof. Erich Poppitz